Ultrasonic Tourniquet

ABSTRACT

A system for controlling blood flow through a zone of a patient limb, the zone being bounded by a proximal end and a distal end, comprises: a cuff configured for securing to the limb and for covering the zone, the cuff providing pressure to the zone for occluding the flow of blood flowing through the zone in the direction from the proximal to distal ends of the zone; an array of sensors fitting between the cuff and the limb and arranged for sensing and signaling the amount of penetration of blood flow into the zone; and a control instrument connected to the array and to the cuff for regulating the pressure in the cuff to occlude the blood flow in the zone depending upon the amount of blood flow penetration signaled by the array.

FIELD OF THE INVENTION

This invention pertains to tourniquet systems commonly used for stoppingarterial blood flow into a portion of a surgical patient's limb tofacilitate the performance of a surgical procedure, and for emergencyand military applications.

BACKGROUND OF THE INVENTION

Typical surgical tourniquet systems of the prior art include atourniquet cuff which encircles the limb of a surgical patient and atourniquet instrument which is releasably connected to an inflatablebladder within the tourniquet cuff through a length of tubing, therebyestablishing a gas-tight passageway between the cuff and the tourniquetinstrument. The tourniquet instrument contains a pressurized gas sourcewhich is used to inflate and regulate the pressure in the tourniquetcuff above a minimum pressure required to stop arterial blood flowdistal to the cuff, for a duration suitably long for the performance ofa surgical procedure. Many types of surgical tourniquet systems havebeen described in the prior art, such as those described by McEwen inU.S. Pat. No. 4,469,099, U.S. Pat. No. 4,479,494, U.S. Pat. No.5,439,477 and McEwen and Jameson in U.S. Pat. No. 5,556,415 and U.S.Pat. No. 5,855,589.

Many studies published in the surgical literature have shown that thesafest tourniquet pressure is the lowest pressure that will stop theflow of arterial blood past a specific cuff applied to a specificpatient for the duration of that patient's surgery. Such studies haveshown that higher tourniquet pressures are associated with higher risksof tourniquet-related injuries to the patient. Therefore, when atourniquet is used in surgery, surgical staff generally try to use thelowest tourniquet pressure that in their judgment is safely possible. Itis well established in the medical literature that the optimal guidelinefor setting the pressure of a constant-pressure tourniquet is based on“Limb Occlusion Pressure” (LOP). LOP can be defined as the minimumpressure required, at a specific time in a specific tourniquet cuffapplied to a specific patient's limb at a specific location, to stop theflow of arterial blood into the limb distal to the cuff. The currentlyestablished guideline for setting tourniquet pressure based on LOP isthat an additional safety margin of pressure is added to the measuredLOP, to account for physiologic variations and other changes that may beanticipated to occur normally over the duration of a surgical procedure.Surgical tourniquet systems of the prior art that can automaticallymeasure the LOP of individual surgical patients and that canautomatically recommend a tourniquet pressure setting based on measuredLOP are described by McEwen in U.S. Pat. No. 5,439,477, by McEwen andJameson in U.S. Pat. No. 5,556,415, and by McEwen et al. in U.S. Pat.App. No. 20060253150.

Standard cylindrical tourniquet cuffs are ideally suited for applicationto patients with cylindrical limbs. However, when applied to a patientwith a tapered limb, a cylindrical cuff will not optimally match thelimb taper, and will typically result in a snug fit proximally and aloose fit distally. Consequently, a cylindrical cuff may prove unable toachieve a bloodless field distal to the cuff at normal pressures or mayrequire a substantially higher and more hazardous inflation pressure toachieve a bloodless field, and when inflated may have a tendency to rollor slide distally on the limb during a surgical procedure. In an effortto match the taper of a patient's limb at a desired cuff location, sometourniquet cuffs of the prior art are designed to have an arcuate shape,and are commonly called contour cuffs. When a contour cuff surrounds alimb having a matching taper, a uniformly snug fit can be achievedbetween the cuff and the limb from the proximal to distal cuff edges.Wide contour tourniquet cuffs of the prior art have been shown in thesurgical literature to substantially reduce pressures required to createa bloodless surgical field distal to the inflated cuff (Younger et al.,‘Wide Contoured Thigh Cuffs and Automated Limb Occlusion MeasurementAllow Lower Tourniquet Pressures’, Clin Orthop 428:286-293, 2004). Lowertourniquet pressures are associated in the surgical literature withlower risk of injuries to surgical patients.

In military combat and emergency situations, loss of blood is a majorcause of death if the injured person is alone or when medical assistanceis not immediately available. The use of a tourniquet to stop blood lossfrom an injured arm or leg is a well-known technique for preventingdeath in these situations. Once the primary objective of preventingdeath due to blood loss is achieved, it is desirable to prevent furtherinjury to the limb due to excessive pressure and time of tourniquetapplication. To minimize mechanical injury to the blood vessels andother soft tissues beneath the tourniquet, the pressure applied by thetourniquet to the underlying arterial blood vessels should be onlyslightly higher than the minimum pressure required to stop blood flow inthe vessels. A prior-art tourniquet that has been widely used inmilitary and emergency applications is described by McEwen et al. inU.S. Pat. No. 6,746,470.

There is a need for a tourniquet system that can accurately and reliablysense arterial blood flow in a limb near a tourniquet cuff thatencircles the limb. This needed tourniquet system would allow blood flownear the tourniquet cuff to be sensed and continuously monitored so thatcuff pressure could be maintained near the minimum pressure required tostop blood flow past the cuff, thus increasing safety in surgery and foremergency and military applications.

There is a related need for a tourniquet system that can apply aselected pressure to a selected portion of a limb beneath a tourniquetcuff in response to blood flow sensed in the portion of the limb. Thisneeded system would allow the application of a level of pressure to theselected portion that is sufficient to stop blood flow in the selectedportion without requiring the application of the same level of pressureto other portions of the limb beneath the tourniquet cuff, therebyincreasing the precision of pressure application and improving safety.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a pictorial representation of the preferred embodiment in asurgical application.

FIG. 2 shows the stretchable sleeve of the preferred embodiment.

FIG. 3 depicts the contour cuff portion of the preferred embodiment.

FIG. 4 is a block diagram of the instrument portion of the preferredembodiment.

FIG. 5 is alternate form of the preferred embodiment

FIG. 6 depicts the cuff portion of the alternate embodiment.

FIG. 7 depicts detection of blood flow by the alternate embodiment.

FIGS. 8 a and 8 b depict the operation of an auxiliary bladder of thealternate embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The embodiment illustrated is not intended to be exhaustive or limit theinvention to the precise form disclosed. It is chosen and described inorder to explain the principles of the invention and its application andpractical use, and thereby enable others skilled in the art to utilizethe invention.

FIG. 1 depicts the preferred embodiment in use applied to limb 2 of apatient. Stretchable sleeve 4 is shown applied along a portion of thelength of limb 2. Gel pad 6 forms part of sleeve 4 and provides a meansfor the efficient coupling of high-frequency acoustic energy betweenultrasonic transducer array 8 and the tissues of limb 2. Ultrasonictransducer array 8 is connected to instrument 10 via cable 12. Contourtourniquet cuff 14 is applied over sleeve 4 and transducer array 8 toencircle limb 2 at a location desired by a user. This configurationpermits ultrasonic transducer array 8 to insonify a volume of tissuewithin a zone 16 of limb 2 bounded by a proximal end 18 and a distal end20, as shown in FIGS. 1 and 2, thereby permitting instrument 10 tomeasure the flow of blood in arteries within zone 16 via the Dopplereffect and to detect changes in blood flow that may occur in response tothe pressurization of contour cuff 14. As described further belowinstrument 10 may be configured to control the pressures applied bycontour cuff 14 to limb 2 to maintain the blood flow in the insonifiedarteries at predetermined levels or at levels set by a user ofinstrument 10.

Ultrasonic transducer array 8 comprises one or more arrays ofpiezoelectric crystal elements or capacitance micromachined ultrasonictransducer cells or other materials and technologies known in the art tobe suitable for transmitting and receiving high frequency acousticenergy. By adjusting the relative phases of electronic signals appliedto the crystal elements that comprise an array the ultrasound wavesproduced by the array may be steered and focused to insonify a selectedregion within zone 16. Instrument 10 operates ultrasonic transducerarray 8 to scan volumes of tissue to detect arterial blood flow.Ultrasonic waves are emitted by transducer 8 at scanning angles relativeto the surface of transducer 8 and traverse the tissue underlyingtransducer 8. The waves emitted by transducer 8 reflect off varioustissue structures within the limb, Doppler frequency shifts in thereflections indicate moving structures, such as flowing blood. Regionswhere blood flow is detected may be localized in terms and scan angleand tissue depth and quantified by analyzing the magnitude of theDoppler frequency shifts of the returned echoes.

FIG. 2 depicts stretchable sleeve 4 of the preferred embodiment appliedto limb 2. Sleeve 4 has a tubular and elastically stretchable shape andis designed for use with tourniquet contour cuff 14 and to conform tolimbs of varying tapers. Gel pad 6 forms part of sleeve 4 and iscomprised of ultrasound coupling gel contained within a thinpolyurethane membrane or other material transparent to ultrasound. Gelpad 6 may also be comprised of other conformable materials or substancessuitable for coupling ultrasonic transducer 8 to limb 2. In thepreferred embodiment, ultrasonic transducer array 8 forms part of gelpad 6; however, it will be apparent however that ultrasonic transducerarray 8 could be independent of gel pad 6 and interposed between contourcuff 14 and gel pad 6 of sleeve 4. For clarity only a single gel pad 6and transducer array 8 have been shown and described; however, it willbe apparent that sleeve 4 may contain multiple gel pads with transducerarrays spaced circumferentially around the sleeve and that sleeve 4could be formed entirely of a stretchable ultrasound conductingsubstance or materials. Sleeve 4 may include one or more gel padssuitable for the coupling of high-frequency acoustic energy between thetissues of limb 2 and ultrasonic transducer array 8. Sleeve 4 has atubular and elastically stretchable shape, with an un-stretchedcircumference of less than the minimum limb circumference recommendedfor contour cuff 14 so that it may be elastically stretched when appliedto a limb having the minimum recommended circumference. Sleeve 4 may beelastically stretched in a radial direction to a circumference at leastequal to the maximum limb circumference recommended for contour cuff 14.When elastically stretched and applied to a limb having the maximumrecommended circumference, the maximum pressure applied to theunderlying limb by sleeve 4 may be less than the pressure required toocclude venous blood flow in the limb. When applied to a limb having themaximum recommended circumference, the length of the gel pad withinsleeve 4 is not less than width of contour cuff 14. When elasticallystretched and applied to a limb having the minimum recommendedcircumference, the pressure applied to the underlying limb by sleeve 4is sufficient to maintain sleeve 4 and gel pad 6 in full contact withthe limb throughout the length of sleeve 4 and gel pad 6.

Contour cuff 14 is similar in design and construction to the cuffdescribed by McEwen et al. in U.S. patent application Ser. No.11/384695. As shown in FIG. 3 contour cuff 14 has a substantiallyarcuate shape with the width of the cuff reduced near the end edges. Thearcuate shape of cuff 14 and the degree to which the width near the endedges is reduced are predetermined to allow cuff 14 to be applied tolimbs with a predetermined range of tapers such that contour cuff 14remains substantially in contact with the limb along its width aroundthe circumference of the limb. When contour cuff 14 is correctly appliedto a patient limb as shown in FIG. 1 the side edge of contour cuff 14with the greater radius is proximal and the side edge with the lesserradius is distal on the limb.

Contour cuff 14 is secured around the limb by securing straps 22 and 24.Securing straps 22 and 24 are non-releasably attached to a non-inflatingregion of contour cuff 14 near an end edge. Securing straps 22 and 24have fastening portions which releasably engage with the outer surfaceof cuff 14 and bending portions which permit the fastening portions tobe positioned such that they can completely engage the outer surfacewithin the side edges of contour cuff 14. In the preferred embodimentthe outer surface of contour cuff 14 and the fastening portions ofsecuring straps 22 and 24 are formed from Velcro-type materials. Theouter surface of cuff 14 is a loop type material and the fasteningportions of securing straps 16 and 18 are formed from hook typematerial. Tie strap 26 shown in FIG. 3, provides a means for the user toalign and pull cuff 14 snug around limb 2. When contour cuff 14 has beensecured around limb 2 the ends of tie strap 26 may be tied together tohelp maintain the overlapping portion of the cuff in alignment aroundlimb 2 by preventing the cuff from twisting, telescoping and rolling onthe limb when pressurized. For clarity, tie strap 26 is not shown inFIG. 1.

Contour cuff 14 has two independent cuff bladders, proximal cuff bladder28 and distal cuff bladder 30; the perimeter 32 of the bladders isdepicted in the top view of contour cuff 14 shown in FIG. 3. Theproximal and distal cuff bladders are pressurized with fluid frominstrument 10 to apply pressure to proximal and distal portions of limb2 underlying cuff 14. In the preferred embodiment the fluid used topressurize (inflate) the cuff bladders of contour cuff 14 is air, itwill be apparent that other fluids could be used, for example the fluidcould be another gas such as nitrogen or the fluid could a liquid suchas water. Air from instrument 10 is supplied to proximal bladder 28 viatubing 34 and bladder port 36, fluid is supplied to distal bladder 30via tubing 38 and bladder port 40. Contour cuff 14 is known in the artas single port cuff, it will be apparent that cuff 14 could beconfigured as a dual port and instrument 10 adapted with separateconnections to each bladder for the supply of fluid to the bladder andthe measurement of pressure within the bladder.

As can be seen in FIG. 1, instrument 10 has a user interface consistingof graphic display panel 42 and keypad 44. Display panel 42 is employedfor the selective display of information in alphanumeric and graphicalformats that may include: measured pressure levels in proximal bladder28 and distal bladder 30, reference or desired pressure levels inproximal bladder 28 and distal bladder 30, blood flow measurements fromtransducer 8, and other information required for the operation ofinstrument 10.

Keypad 44 provides a means for a user of instrument 10 to control theoperation of instrument 10.

Referring to the block diagram of instrument 10 shown in FIG. 4,controller 46 comprises a microcomputer, associated memory and controlsoftware, analog and digital peripheral interface circuitry, and othernecessary support components for the operation of instrument 10.

As shown in FIG. 4, pneumatic pump 48 is pneumatically connected toreservoir 50. In response to control signals from controller 46, pump 48operates to pressurize reservoir 50. Reservoir pressure transducer 52 ispneumatically connected to reservoir 50 and generates a reservoirpressure signal. The reservoir pressure signal is communicated tocontroller 46. Controller 46 acts to maintain the pressure in reservoir50 near a predetermined or calculated reservoir pressure level.

Instrument 10 is shown configured with two pressure regulators, firstpressure regulator 54 and second pressure regulator 56. First pressureregulator 56 maintains the pressure in proximal cuff bladder 28 near thelevel of the first cuff pressure reference level communicated fromcontroller 46. Second pressure regulator 56 maintains the pressure indistal cuff bladder 30 near the level of the second cuff pressurereference level communicated from controller 46. Although instrument 10has been described with two pressure regulators it will be apparent thatadditional pressure regulators could be included within instrument 10 toindependently control the pressure in multiple cuff bladders to applydiffering selected pressures to various selected portions of a limb.

Ultrasonic transducer interface 58 is the ultrasound engine ofinstrument 10 and includes transceivers for driving and receivingsignals from the elements of transducer array 8 and electronics for beamforming, steering, focusing, signal amplification, filtering, and signalprocessing functions. Interface 58, under the direction of control ofsignals from controller 46, acts to scan volumes of tissue within zone16 to produce signals indicative of blood flow for communication toblood flow detection module 60.

Blood flow detection module 60 receives signals from ultrasonictransducer interface 58 indicative of scan position and Doppler bloodflow magnitudes. The module uses this information to detect the volumesof tissue within the scanning area of transducer 8 where blood flow ispresent and to quantify and localize the blood flow information forcommunication to controller 46 and display to a user.

Signals from blood flow detection module 60 may be used by controller 46to perform automatic measurements of limb occlusion pressure (LOP). LOPis the minimum pressure required, at a specific time in a specifictourniquet cuff applied to a specific patient's limb at a specificlocation, to stop the flow of arterial blood into the limb distal to thecuff Controller 46 of instrument 10, in response to user input viakeypad 44, may measure LOP by: first automatically scanning for arterialblood flow in the tissue of limb 2 immediately distal to contour cuff14, then after localizing and quantifying the blood flow automaticallyincreasing the second cuff reference pressure level to increase thepressure applied by distal cuff bladder 30 to the tissues of limb 2until the blood flow detected distal to contour cuff 14 falls below apredetermined minimum flow threshold, or until the pressure in distalcuff bladder exceeds a predetermined maximum bladder pressure. Thepressure level within bladder 30 at which the detected blood flow distalto the contour cuff 14 first falls below the predetermined minimum flowthreshold is the measured LOP for distal bladder 30. Controller 46 canact similarly to determine the minimum pressure required in proximalcuff bladder 28 to prevent blood flow past bladder 28.

A measurement of LOP as described above is valid only at the time themeasurement is made, and physiological parameters that change over timesuch as blood pressure may cause changes in LOP over time. Toautomatically maintain the minimum pressures required in proximalbladder 28 and distal bladder 30 to prevent blood flow past cuff 14 overa period of time when physiologic parameters may vary, such as during atime period suitably long for surgery, instrument 10 operates asfollows: a) first the LOP for proximal bladder 28 and distal bladder 30is automatically determined, as described above, and the bladders arepressurized by setting the first and second reference pressure levels tothe respective LOP values measured for each cuff bladder; b) the limbtissues insonified by ultrasonic transducer 8 are continuously scannedto localize and quantify blood flow within zone 16 which includes tissuevolumes proximal and distal and beneath contour cuff 14; c) controller46 continuously measures the LOP of proximal bladder 28 by firstdecreasing the first reference pressure level until the detected bloodflow past proximal bladder 28 is above a predetermined flow thresholdand then increasing the first reference pressure level until detectedblood past proximal bladder 28 is below a predetermined flow threshold;d) the second reference pressure level, and thereby the pressure appliedto limb 2 by distal cuff bladder 30, is automatically adjusted toreflect any change in the value of LOP measured for proximal bladder 28(for example, if the minimum pressure required to occlude blood flowpast proximal bladder 28 decreases by 15 mmHg then the pressure appliedby distal bladder 30 is reduced by 15 mmHg, and conversely if thepressure required to occlude blood flow past proximal bladder 28increases by 15 mmHg then the pressure applied by distal bladder 30 isincreased by 15 mmHg); and e) if at any time blood flow is detecteddistal to cuff 14 then controller 46 may incrementally increase thepressure level in distal bladder 30 by predetermined amounts until thedetected flow stops.

In another mode of operation, instrument 10 may operate to continuouslymaintain the minimum pressures necessary to occlude blood flow distallypast contour cuff 14. This is accomplished by first automaticallyincreasing the pressure applied by distal bladder 30 and proximalbladder 28 until blood flow sensed beneath the cuff is less than apredetermined minimum. Then, pressure is continuously varied byincrementally increasing and decreasing the pressure applied by proximalbladder 28 to maintain the rate of blood flow sensed beneath contourcuff 14 below and near the predetermined minimum threshold. In this modeof operation, the pressure in distal bladder 30 is automaticallyadjusted to reflect any change in the pressure required in the proximalbladder 28 to occlude blood flow beneath contour cuff 14.

FIGS. 5, 6, 7, 8 a and 8 b depict an alternate embodiment of theinvention. In FIG. 5 emergency and military tourniquet (EMT) cuff 62 isshown applied to patient limb 64. EMT cuff 62 is similar in design andconstruction to the cuff described in U.S. Pat. No. 6,746,470 and isadapted for rapid application to a limb in an emergency or lifethreatening situation such as military combat. Bladder 66 is fixed atone end to EMT cuff clamp 68. To apply cuff 56 to a limb bladder 66encircles the limb and the free end of bladder 66 passes through thesealing surfaces of EMT cuff clamp 68. EMT cuff clamp 68 retains cuffbladder 66 around limb 64 and seals the encircling bladder 66 across itswidth. The width of the sealing surfaces of clamp 68 are selected to begreater than the width of bladder 66 thereby allowing bladder 66 to besealed across its width at angles other than 90 degrees relative to thebladder edge, such as is the case when bladder 66 assumes a conicalshape when wrapped around a conical limb (such as a typical thigh). Thisis important in achieving a snug fit around limbs of various degrees ofconical shape.

The limb facing surface of EMT cuff clamp 68 includes ultrasonictransducer array 70 which communicates with instrument 10 via cable 72.Ultrasonic transducer array 70 is similar to transducer array 8described above and is shown in FIGS. 6 and 7. Ultrasound coupling gelor a gel pad may be placed between the face of ultrasonic transducerarray 70 and limb 64.

EMT cuff 62 is preferably applied to a limb such that clamp 68 withtransducer array 70 is positioned over the major artery in the limb, forexample when applied to the upper arm the camp is placed over thebrachial artery or when applied to the upper thigh the clamp is placedover femoral artery. As shown in FIG. 7 ultrasonic transducer array 70insonifies a volume of tissue within zone 74 of limb 64. Zone 74 isbounded by a proximal end 76 and a distal end 78. Ultrasound transducerarray 70 is used to measure blood flow in the portion of major artery 80that lies within zone 74. Changes in blood flow in major artery 80 inresponse to pressures applied to the limb by EMT cuff 62 can be detectedby ultrasound transducer array 70. In FIG. 7 the direction of blood flow82 in the major artery 80 is shown and the shaded area of major artery80 represents penetration of blood flow beneath EMT cuff 62.

Encircling cuff bladder 66 is pressurized by first pressure regulator 54of instrument 10 via tubing 84 and bladder port 86. As shown in FIGS. 5,6, 8 a and 8 b EMT cuff 62 includes an auxiliary bladder 88 which islocated on the limb facing surface of encircling bladder 66. Auxiliarybladder 88 is pressurized by second pressure regulator 56 of instrument2 via tubing 90 and bladder port 92. In the preferred embodiment thewidth of auxiliary bladder 88 is no greater than the width of encirclingbladder 66 and the length of auxiliary bladder is less than theinflating portion of encircling bladder 66 when secured around a limb byclamp 68. Auxiliary bladder 88 is pressurized by instrument 10 to extendthe reach of encircling bladder 66 in a predetermined location toocclude blood flow, as detected by ultrasonic transducer array 70, inarteries lying deep within the tissues of limb 64 (within a portion ofzone 74) underlying auxiliary bladder 88. This configuration ofoverlapping cuff bladders allows for the selective application ofpressure to occlude deep arteries without increasing the overallpressure applied to the limb. It will be appreciated that to applyadditional or supplemental pressure to other portions of limb 64encircled by EMT cuff 62 additional auxiliary bladders of varying shapeand size may be selected to form part of EMT cuff 62, and that thesebladders may not necessarily overlap with encircling bladder 66.

The invention may automatically maintain the pressures applied to a limbby EMT cuff 62 at the minimum level required to prevent blood flowdistal to the location of the cuff on the limb. Typically, a user firstapplies ultrasound coupling gel or a gel pad to the transducer surfacesof EMT clamp 68. Next, EMT cuff 62 is applied to a limb 64 at a desiredlocation by wrapping encircling bladder 66 around the limb at thelocation; the end of bladder 66 is pulled through clamp 68; clamp 68 isapproximately aligned above the major limb artery 80; and clamp 68 issecured to maintain bladder 66 snugly around the limb and clamp 68 incontact with the tissues of limb.

Instrument 10 is then directed via keypad 44 to automatically scan forarterial blood flow in the limb tissues underlying clamp 68 and toincrease the pressure in encircling bladder 66 until a predeterminedpressure threshold is reached or until the rate of blood flow distal tothe midline 94 of EMT cuff 56 is equal to or less than a predeterminedminimum flow threshold. If the predetermined pressure threshold isreached in encircling bladder 66 and blood flow distal to the midline ofEMT cuff 62 remains above the predetermined minimum flow threshold,instrument 10 may act to pressurize auxiliary bladder 88 to occludearteries lying deeper beneath EMT cuff 62. To maintain the pressure inencircling bladder 66 and auxiliary bladder 88 at the minimum necessaryto prevent blood flow distal to EMT cuff 62, instrument 10 continues tomonitor blood flow in limb tissues proximal to the midline 94 of EMTcuff. Instrument 10 continues to scan for arterial blood flow in tissuesunderlying clamp 68 and monitors the distance that blood flow penetratesbeneath the cuff from the proximal edge towards the midline. Instrument10 automatically adjusts the pressure within bladders 66 and 88 withinpredetermined limits to prevent blood flow from penetrating beyond themidline 94 of EMT cuff 62, thereby maintaining the pressures in EMT cuff62 at the lowest level necessary to occlude blood flow past EMT cuff 62.

It will be apparent that instrument 10 may be used to monitor blood flowbeneath a cuff, and generate alarms perceivable by a user if the bloodflow sensed beneath the cuff exceeds a predetermined penetrationthreshold. For example, an alarm may be generated if the sensed bloodflow past the midline of the cuff exceeds a predetermined threshold, oran alarm may be generated to indicate that sensed blood flow beneath thecuff is nearing the midline of the cuff and thus alert the user to thepossibility that blood flow past the cuff may be imminent, These alarmsmay be operable regardless of whether the cuff pressure level wasestablished automatically as described above, or was established by userinput to instrument 10, or was established manually by application ofthe cuff in an emergency or military setting.

Auxiliary bladder 88 of EMT cuff 62 may be adapted as described above toprovide EMT cuff 62 with additional reach in a selected area or may beadapted as described below and shown in FIGS. 8 a and 8 b to extend thevolume of tissue that may be scanned by transducer array 70. The maximumvolume of tissue that can be insonified by transducer array 70 to detectblood flow is determined by the geometry and other physical andelectrical characteristics of transducer array 70. The face of clamp 68containing transducer array 70 is normally tangent to the surface oflimb 64. By varying the angle of the face of transducer array 70 withrespect to the surface of the limb additional tissue volumes may bescanned for blood flow. This would be much like a user varying the anglea hand held ultrasound probe held parallel to the limb to search forarterial blood flow. Auxiliary bladder 88 may be positioned close toclamp 68 and act as a “transducer scanning” bladder so that when it ispressurized to a pressure above that of encircling bladder 66 it willinflate against the limb and cause clamp 68 to tilt relative to the limbsurface as shown in FIG. 8 b. Instrument 10, by varying the pressurewithin auxiliary bladder 88, can mechanically scan ultrasonic transducerarray 70 to extend the volume of tissue in which blood can be detected.This simple low-cost mechanical scanning of ultrasonic transducer array70 may permit greater tolerance in the application of clamp 68 to thelimb with respect to the position of the clamp relative to the majorarteries of the limb. It will be apparent that additional auxiliarytransducer scanning bladders (as shown, for example, in dashed lines at89, FIG. 6) may form part of cuff 62 to provide mechanical scanning inmore than one direction and that scanning bladders may form part ofcontour cuff 14 described above, and be configured to control the angleof the face of ultrasonic transducer 8 with respect to limb 2.

We claim:
 1. A tourniquet for controlling blood flow through a zone of apatient limb, the zone being bounded by a proximal end and a distal end,comprising: a cuff configured for securing to the limb and for coveringthe zone, the cuff being adapted to apply pressure to the zone foroccluding the flow of blood flowing through the zone in the directionfrom the proximal to distal ends of the zone; an array of sensorsarranged for insonifying a first volume of tissue in the zone andsignaling a distance from the proximal end that the arterial blood flowpenetrates into the zone; a control instrument connected to the arrayand to the cuff for regulating the pressure applied by the cuff toocclude the blood flow in the zone depending upon the arterial bloodflow penetration distance signaled by the array; and scanning meansadapted for varying the sensors while the zone is covered with the cuffsecured to the limb to apply pressure to the zone, thereby to insonify asecond volume of tissue in the zone.
 2. The system of claim 1 whereinthe control instrument establishes a threshold penetration of blood flowand is operable for occluding blood flow in the zone when the arterialblood flow penetration distance signaled by the array exceeds thatthreshold.
 3. The system of claim 2 wherein the threshold is establishedto be the midline between the proximal and distal ends.
 4. The system ofclaim 1 further comprising a clamp to which the array is mounted, theclamp also configured for securing the cuff in position on the limb forcovering the zone.
 5. The system of claim 1 further comprising aninflatable auxiliary bladder disposed adjacent to the cuff andseparately operable by the control instrument to supplement the pressureapplied to a portion of the zone by the cuff.
 6. A method forcontrolling blood flow through a zone of a patient limb, the zone beingbounded by a proximal end and distal end, comprising the steps of:covering the zone with a cuff secured to the limb to provide pressure tothe zone for occluding the flow of blood flowing through the zone in thedirection from the proximal to the distal ends of the zone; insonifyinga first volume of tissue in the zone for sensing a distance ofpenetration of arterial blood flow into the zone from the proximal end;signaling the distance from the proximal end that the arterial bloodflow penetrates into the zone; regulating the pressure in the cuff toocclude the blood flow in the zone depending upon the distance signaled;and insonifying a second volume of tissue in the zone for sensing adistance of penetration of arterial blood flow into the zone from theproximal end.
 7. The method of claim 6 including the steps of:establishing a threshold penetration of blood flow; and occluding bloodflow in the zone when the arterial blood flow penetration distancesignaled exceeds that threshold.
 8. The method of claim 7 including thestep of establishing the threshold to be the midline between theproximal and distal ends.
 9. The method of claim 6 including the stepof: mounting to a clamp sensors for sensing and signaling the distanceof penetration of arterial blood flow into the zone from the proximalend; and using the clamp to secure the cuff to the limb.
 10. The methodof claim 6 including the step of pressuring a portion of the zone tosupplement the pressure provided by the cuff to the zone.